Baba Basra 159a - Witnesses in Contracts who become Passul

The Shulchan Aruch (C.M. 36:34-35) points out that if a person is signed in a contract and later becomes passul for eidus, there is a distinction between becoming passul as a gazlan or becoming passul as a relative. If Reuven is signed in a contract and later becomes a gazlan, he can no longer confirm his own signature since he is no longer kasher for eidus, but others can confirm his signature. However, the two witnesses who are confirming his signature must know that he signed prior to becoming a gazlan because otherwise we are concerned that the contract was forged and only signed after he was already passul. The source of this is Tosafos in our sugya who explains the requirement of כגון שהוחזק כזת ידו בבית דין to mean that the beis din has to know that he signed the contract before becoming a gazlan (not a henpek, which is a kiyum written beneath the signatures as the Rashbam explains). But, if reuven signs a contract as a witness and later becomes a relative of the person who he signed for (such as becoming a son in law by marrying his daughter), he cannot confirm his own signature as a relative. However, if there are two witnesses who can confirm his signature, they don't have to know that he signed prior to becoming a relative, because we are not concerned that he forged the contract or signed b'issur. The source of this distinction is clear in the gemara which says that for becoming a relative the contract is valid אע"פ שלא הוחזק כתב ידו בבית דין. The gemara explains that since the p'sul of a relative isn't a concern of lying, rather a geziras hakasuv, there is no concern that the contract was forged.

What about if a witness who signed in a contract later becomes no'geiah, meaning that they have monetary gain by the contract. Must the witnesses who are confirming his signature be sure that the contract was signed prior to him becoming a no'geiah? Are we concerned that a no'geiah would forge the contract just as a gazlan would, or is he more similar to a relative where we don't have such a concern? The Pischei Teshuva (46:11) quotes from Teshuvos Na'os Desheh that this is explicit in our gemara. The gemara speaks of a case where Reuven signed in a shtar on something that later falls to him as inheritance so that he becomes a no'geiah. The gemara insists that when witnesses confirm his signature that it be הוחזק כתב ידו בבית דין. Since a no'geiah is also a concern of being me'shaker, we consider him like a gazlan who would forge the contract and therefore require that they know he signed it before becoming a no'geiah.

Based on this approach that a no'geiah is a concern of being me'shaker like a gazlan, the gemara is difficult. How can the gemara prove that the p'sul of a relative is a gezeiras hakasuv - דאי לא תימא הכי משה ואהרן לחותנם משום דלא מהימני הוא - meaning that by moshe and aharon there is certainly no concern of being me'shaker, so it must be a gezeiras hakasuv. Since no'geiah b'eidus is a concern of being me'shaker rather than a gezeiras hakasuv, it should come out that there is no p'sul of no'geiah by people like moshe and aharon since they would certainly not have a chashash me'shaker? If we assume that no'geiah would invalidate moshe and aharon because it is a gezeiras hakasuv to treat them as if they would lie (even though we aren't really concerned that they would lie), we can say the same thing for the p'sul of being a relative and require the eidim who confirm the signature of a relative to know that he signed the contract prior to becoming a relative? B'kitzur, since no'geiah is a concern that he is not telling the truth, this p'sul shouldn't apply to people who we know are honest such as moshe and aharon?